Most metazoan cells can be classified as either epithelial or mesenchymal based on morphology, behavior and molecular signatures. Epithelial tumor cells can become mesenchymal cells and vice versa via phenotypic transitions, a process known as epithelial plasticity. Epithelial cells are generally polar in the apico-basal direction, adherent to adjacent cells in the plane perpendicular to the polarity, and non-motile in the polar direction. Mesenchymal cells, in contrast, lack polarity, do not form tight interactions with neighboring cells, and are motile. In adult animals, epithelial and mesenchymal cells remain stably in one state or the other; that is, an epithelial cell does not change its properties and become mesenchymal. During development, however, epithelial cells of the early embryo give rise to all three embryonal layers (endoderm, mesoderm and ectoderm), which include mesenchymal cells (Hay, E. D., et al. Am. J. Kidney Dis. 1995, 26, 678-690). Therefore, these early embryonal cells have the ability to transition between epithelial and mesenchymal states. Embryos have been shown to undergo epithelial-mesenchymal transitions (EMTs) as well as mesenchymal-epithelial transitions (METs) (Acloque, H., et al. J. Clin. Invest. 2009, 119, 1438-1449).
Epithelial plasticity (EP) refers to the reversible loss of the epithelial cellular phenotype, a process known to occur during cancer metastasis. This EP biology has been linked in multiple studies to the risk of cancer metastasis and the acquisition of mesenchymal and/or stemness properties through the EMT process. EMT has been linked to chemoresistance, invasion, intravasation, and dissemination in multiple preclinical models of cancer. The MET process, which results in the re-expression of the epithelial phenotype, is also likely of great importance in development and metastasis and has been linked to metastatic colonization and survival of tumor cells in the metastatic niche. For example, in prostate cancer, mesenchymal biomarkers may be upregulated during androgen deprivation in prostate cancer cell lines, animal models, and in patient tumor specimens. Moreover, these biomarkers are plastic, revert upon replacement of testosterone, and are linked to an increased metastatic propensity and chemoresistance. Mesenchymal-like tumor cells may better promote local tumor invasion and intravasation/extravasation, but epithelial tumor cells may be necessary for eventual survival and proliferation in the metastatic niche, illustrating the potential relevance of the dual nature of EP in mediating the full process of metastasis.
Circulating tumor cells (CTCs), which are cells that have detached from a primary tumor and circulate in the bloodstream, have potential prognostic, predictive and surrogate implications in oncology. CTCs may constitute seeds for subsequent growth of additional tumors (metastasis) in different tissues. Thus, detection of CTCs can provide a diagnosis and/or prognosis for overall survival and therapeutic implications in subjects with cancers such as metastatic prostate and breast cancer. The number of CTCs in any patient sample (e.g., a blood sample) can be very small, which can make detection difficult. Current methods for detecting CTCs are based on the detection of epithelial cell adhesion molecule (EpCAM) expression, which is a biomarker associated with epithelial cells. However, during the process of metastasis, circulating tumor cells (CTCs) may lose their epithelial phenotype and acquire a mesenchymal phenotype that is not sufficiently captured by existing epithelial-based CTC technologies. During metastasis, tumor cells may exist as a spectrum of epithelial to mesenchymal phenotypes. CTCs may lose their epithelial phenotype and acquire a mesenchymal phenotype, which may not be captured with existing epithelial-based CTC technology and thus lead to the under-detection of CTCs under circumstances where cells undergo a decrease or loss of EpCAM expression, such as during biologic processes including EMT. Because of the role CTCs can play in the diagnosis, monitoring, and prognosis of disease in patients having cancer, any shortcoming in the detection technology needs to be addressed by the art.
There is recent evidence to suggest that CTCs with a mesenchymal phenotype are missed by CELLSEARCH® and other epithelial-based technologies. Accordingly, there is a need for methods and systems for capturing CTCs that do not rely on existing capture technologies, and methods for correlating CTC detection to diagnosis, monitoring, and prognosis of disease in cancer patients.